The present invention relates to telephone systems and more particularly to the detection ring signals on an incoming telephone line. The invention is particularly beneficial in the context of digital signal processor (DSP) based telephone devices, where processing power is at a premium.
In typical telephone systems, a ring signal or ring tone is generated by a called party's central office and transmitted along a telephone line to a local loop subscriber, to alert the subscriber of an incoming call. The telephone line is usually a twisted pair of copper wires, but may take other forms as well, including, but not limited to, fiber optics, microwave and other wired or wireless connections. Historically, ring signals on these lines were direct current (DC) signals of sufficient voltage to cause analog telephones to ring. By modem convention, a ring signal is an alternating current (AC) signal having a frequency in the range of 14 Hz to 65 Hz (usually about 20 Hz) and an amplitude of at least 22 Volts rms.
In general, any device designed to receive incoming telephone calls must be configured to detect an incoming ring signal while on hook. Modem electronic telephones, for instance, must be able to detect the presence of a ring signal on the incoming phone line in order to recognize when to simulate a ringing sound. Answering machines or computer modems, as additional examples, must be able to detect the presence of a ring signal in order to know when to go off hook to communicate with an incoming call. In any event, these devices must be able to quickly respond to a ring signal that falls within accepted ranges of frequency and amplitude, and to not falsely respond to incoming signals outside of those accepted ranges.
The process of detecting a ring signal is made difficult in part due to the presence of distortions on the incoming telephone line and in the incoming signal. These distortions arise from several sources. Principally, a telephone line naturally carries band limited noise in the range of 300 Hz to 4 kHz. This noise is added to any input signal, including, for instance, a speech signal or a ring signal. In addition, electronic devices connected to public telephone lines in the United States are required by regulation to prevent DC or destructive power surges from entering the network. In most cases, these devices include a line interface circuit having an isolation transformer or other circuitry that is optimized for telephone signals of between 300 Hz and 3.3 kHz and is incapable of passing signals of over 160 Volts peak-to-peak (or about 56 Volts rms). As a consequence, however, the transformer will tend to distort signals having the frequency and/or voltage range of a ring signal, transferring energy from the fundamental harmonic of these signals into second or higher level harmonics.
Modern telephone devices are often DSP based. A DSP chip in these devices may be specially designed and/or programmable and may be configured to perform necessary communication functions, such as tone generation and speech processing, including, for instance, acoustic echo cancellation, modulation and demodulation. Generally speaking, these DSP-based telephone devices detect ring signals by employing either dedicated hard-wired circuitry or complex digital signal processing techniques.
In devices using dedicated hard-wired circuitry to detect ring signals, the ring-detect circuitry typically receives and processes the incoming signal before the signal reaches the isolation transformer and becomes distorted. Such circuitry may include, for instance, a phase locked loop or other mechanism for locking on a signal of a specified frequency. Unfortunately, however, hard-wired circuitry designed to serve a dedicated function such as ring detection is often large and expensive. Additionally, most physical circuit components will degrade as they age, and their performance will diminish. Consequently, such circuitry is not always be able to respond accurately or quickly enough to a proper input signal.
As a result, many DSP-based telephone devices, such as computer modems, are instead configured to detect ring signals through complex digital signal processing techniques. DSP ring detection generally involves passing the incoming signal through a narrow band filter of relatively high Q (selectivity) in search of signals having appreciable energy in the 14 to 65 Hz range, and then subjecting the resulting signal to energy integration and testing code. In this regard, in order to account for the inherent line noise and particularly for the harmonic energy shifts introduced by the line interface circuit, extensive filtering and processing is typically required, sometimes exhausting nearly all of the processing power of the DSP.
Of course, the use of nearly all of the processing power for ring detection may present difficulty in any DSP-based telephone device that must simultaneously detect multiple ringing signals or that must detect a ring signal while simultaneously processing other signals. In a multiple-line phone, for instance, a DSP should be able to detect ring signals simultaneously on all lines. Alternatively, while the DSP is processing speech signals on one line, the DSP should be able to detect ring signals on the other line. Use of substantially all available DSP processing power for ring detection may preclude the inclusion of other commercially desirable features or may necessitate the inclusion of multiple DSP chips, thus increasing manufacturing cost.
In view of these deficiencies, there is a need in the art for a less complex and more robust method of detecting ring signals.